Nooks and crannies

A piece of Mars: This 521×391 m (1709×1283 ft) scene shows a rocky plain with many small impact craters (the bigger ones are ~45 m, or 148 ft across). Dark rippled sand fills the floors of the craters. Why? Once it blows in, it’s hard for the sand to get out. It gets caught in the nooks and crannies of the terrain. The same way it gets caught in your bathing suit and towel at the beach. (ESP_035164_1655, NASA/JPL/Univ. of Arizona)